1. Field of the Invention
The present invention relates to a method for forming a semiconductor device using a crystalline semiconductor in a semiconductor device forming method. Further, the present invention relates to a method for forming thin film transistors (TFTs). TFTs according to the invention can be formed either on an insulating substrate as made of glass or on a semiconductor substrate as made of a single crystal of silicon. More particularly, the invention relates to TFTs formed, utilizing both a crystallization process using thermal and/or optical annealing and an activation process.
2. Description of Related Art
In recent years, an insulated gate semiconductor device comprising a thin film active layer (also called an active region) formed on an insulating substrate has been studied. Especially, researches on insulated gate thin film transistors (TFTs) have been earnestly conducted. These TFTs are formed on a transparent insulating substrate and used to control pixels of a display device such as a liquid crystal display having a matrix structure. Also, the TFTs are used in the driver circuit of the display device. Depending on the material and the state of the crystal of the used semiconductor, they are classified as amorphous silicon TFTs or crystalline silicon TFTs.
Generally, an amorphous semiconductors have small field mobilities and so they cannot be used in TFTs which are required to operate at high speeds. Since the field mobility of P-type amorphous silicon is quite small, it is impossible to form P-channel TFTs (PMOS TFTs). Therefore, a complementary MOS circuit (CMOS) which would be formed by combining an N-channel TFT (NMOS TFT) with such a P-channel TFT cannot be obtained.
On the other hand, crystalline semiconductors have higher field mobilities than those of amorphous semiconductors and thus can operate at higher speeds. With crystalline silicon, PMOS TFTs can be formed as well as NMOS TFTs and so CMOS circuits can be built. For example, in a known active matrix liquid crystal display, not only the active matrix portions but also the peripheral circuit such as drivers are composed of CMOS crystalline TFTs. This structure is known as a monolithic structure. For this reason, TFTs using crystalline silicon have been earnestly studied and developed recently.
One method of obtaining crystalline silicon is irradiation of an amorphous silicon with laser light or other equivalent intense light so as to crystallize the silicon. However, there is no prospect of mass production because of instability of the laser output and instability caused by the fact that the process is quite short.
The method which is currently considered to be capable of being put into practical use is to crystallize amorphous silicon by heat. In this method, crystalline silicon can be obtained with small variations among batches. However, this method is not free from problems.
Usually, in order to obtain crystalline silicon, it is necessary that annealing is carried out at about 600xc2x0 C. for a long time or that annealing is carried out at a high temperature exceeding 1000xc2x0 C. Where the latter method is adopted, the usable substrate material is limited to quartz, thus increasing the cost of the substrate greatly. Where the former method is adopted, the material of the substrate can be selected from various substances but shrinkage of the substrate caused during thermal annealing presents problems. In particular, a decrease in the forming yield due to misalignment of masks has been pointed out. Accordingly, there is a demand for a process at lower temperatures. Specifically, there is a demand for a process which is carried out below the strain points of (preferably at temperatures lower than the strain points of glasses by more than 50xc2x0 C.) various non-alkali glasses used as materials of substrates. The present invention is intended to solve these difficulties. It is an object of the invention to provide a method of mass-producing TFTs without incurring the foregoing problems.
TFTs are formed using a thin film semiconductor formed on a substrate. These TFTs are used in various ICs. Especially, TFTs of this kind have concerned as switching devices located at pixels in an active matrix liquid crystal display and as driver devices formed in peripheral circuit portions.
It is easy to use an amorphous silicon film as thin film transistors used in TFTs. However, this method has the problem that the electrical characteristics are low. In order to improve the characteristics of TFTs, a crystalline thin film silicon may be used. Crystalline silicon films are variously known as polycrystalline silicon, polysilicon, and silicon crystallite. To obtain such a crystalline silicon film, an amorphous silicon film is first formed. Then, the film is crystallized by heating.
However, crystalline silicon thin films obtained by the conventional heating process have relatively small particle diameters, and these particles are not uniform in size. In consequence, their characteristics are not uniform. Furthermore, their mobilities which represents the performance of completed devices are much inferior to the mobilities of single crystal silicon. Therefore, there is a demand for a crystalline silicon thin film having improved characteristics.
Our research has revealed that crystallization can be performed at 450 to 650xc2x0 C., e.g., about 550xc2x0 C., in a short time on the order of 4 hours by depositing a trace amount of elements such as nickel, palladium, and lead on the surface of an amorphous silicon film and then heating the laminate. Also, the obtained crystal grains can be controlled by the temperature and time of the crystallization. This means that an active layer necessary for devices can be formed.
In order to introduce a trace amount of element as described above, or a catalytic element for promoting crystallization, plasma processing, evaporation, or ion implantation is employed. The plasma processing uses a parallel plate type or positive column type CVD apparatus. Electrodes containing a catalytic element are used. A plasma is generated in an ambient of nitrogen, hydrogen, or the like. In this way, the catalytic element is added to the amorphous silicon film.
However, if the above described element exists in abundance in a semiconductor, then the reliability and the electrical stability of an device using this semiconductor is deteriorated. This produces undesirable results.
In particular, an element for promoting crystallization such as nickel (referred to herein as a catalytic element) is necessary to crystallize amorphous silicon but it is desired that the amount of catalytic elements in the crystallized silicon should be reduced to a minimum. To fit this requirement, a catalytic element which tends to be inactive within crystalline silicon is selected. At the same time, the amount of a catalytic element necessary for crystallization is minimized. For this purpose, it is necessary to precisely control the amount of the introduced catalytic element.
Using nickel as a catalytic element, an amorphous silicon film is formed, nickel is introduced by plasma processing and a crystalline silicon film is formed by heating. The crystallization process is carefully examined and discovered the following items:
(1) Where nickel was introduced into the amorphous silicon film by plasma processing, nickel atoms are penetrated considerably deep into the amorphous silicon film before the heating processing.
(2) At first, nuclei of crystals are produced at the surface through which nickel atoms are introduced.
(3) Where a nickel film is formed on the amorphous silicon film by evaporation, crystallization occurs in the same way as in the case in which plasma processing is carried out.
From the above items, it is concluded that all the nickel atoms introduced by plasma processing does not function effectively. That is, if a large amount of nickel is introduced, some nickel atoms may not function sufficiently. Therefore, it is considered that the points or surfaces at which nickel atoms are in contact with silicon atoms function during low temperature crystallization. it is concluded that nickel atoms are required to be dispersed most finely, i.e., almost on an atomic scale. In other words, the requirement is that a minimum concentration of nickel is dispersed on an atomic scale near the surface of an amorphous silicon film within a concentration range which permits low temperature crystallization.
Evaporation can be used as a method of introducing an infinitesimal amount of nickel only into a surface region of an amorphous silicon film, i.e., introducing an infinitesimal amount of a catalytic element for promoting crystallization of an amorphous silicon film only into a surface region of the amorphous silicon film. However, it is not easy to control the evaporation process, and it is difficult to strictly control the amount of the introduced catalytic element.
It is necessary that the amount of the introduced catalytic element is reduced to a minimum. In this case, satisfactory crystallinity cannot be obtained.
It is an object of the present invention to provide a method for forming a semiconductor device using a crystalline semiconductor in a semiconductor device forming method.
Our researches have revealed that crystallization of a substantially amorphous silicon film is promoted by adding a trace amount of catalytic material. Also, the crystallization temperature is lowered, and the crystallization time can be shortened. Examples of the catalytic element include single nickel (Ni), iron (Fe), cobalt (Co) and platinum (Pt), and compounds thereof such as silicides. In particular, these catalytic elements are introduced into the amorphous silicon film by ion implantation or other method. Then, this film is thermally annealed at an appropriate temperature, typically below 580xc2x0 C., to crystallize the amorphous silicon film.
Of course, as the anneal temperature is elevated, the crystallization time is shortened. Also, the concentrations of nickel, iron, cobalt, and platinum are increased, the crystallization temperature is lowered, and the crystallization time is shortened. Our researches have demonstrated that in order to promote crystallization, it is necessary that the concentration of at least one of them is 1017 cmxe2x88x923 or more, preferably in excess of 5xc3x971018 cmxe2x88x923.
Since all the above described catalytic materials are undesirable for silicon, it is desired to suppress their concentrations as low as possible. Our researches have revealed that the total concentration of these catalytic materials is not greater than 1020 cmxe2x88x923.
We took notice of the effects of the catalytic elements and have found that the foregoing problems can be solved by using these effects. In the present invention, the crystallization temperature is lowered by introducing these catalytic elements into amorphous silicon film. The catalytic elements introduced in the silicon film is lowered the temperatures at which the dopant impurities are activated, or recrystallized. We have found that where catalytic elements are uniformly distributed by ion implantation or ion doping before the crystallization, the crystallization progresses quite smoothly. Typically, the crystallization and activation can be sufficiently done below 550xc2x0 C. Also, we have found that if the anneal time is set to less than 8 hours, typically less than 4 hours, then satisfactory results are obtained.
It has been difficult for the prior art thermal annealing to crystallize a silicon film thinner than 1000 xc3x85. In the present invention, the silicon film can be crystallized with great ease, at a lower temperature, and in a shorter time. TFTs having an active region thinner than 1000 xc3x85, especially thinner than 500 xc3x85, have excellent characteristics. In addition, small steps give rise to gate insulating film and gate electrodes having fewer defective step portions. Hence, the forming yield is high. In the past, however, only one method of crystallizing such a thin silicon film is laser annealing because it is difficult to crystallize the film by other methods. The present invention permits crystallization of a thin silicon film by thermal annealing. Furthermore, the production yield can be enhanced for the reason described above. In these respects, the present invention offers an epoch-making technique.
Furthermore, it is an object of the present invention to provide a method of forming a crystalline thin film semiconductor by heating processing using a catalytic element while satisfying the following requirements:
(1) The amount of the introduced catalytic element is controlled and is decreased to a minimum.
(2) The method is made highly productive.
(3) Crystallinity superior to the crystallinity obtained by heating processing is obtained.
In order to satisfy the above described object, a crystalline silicon film is obtained by using the following means. In the present invention, either a single catalytic element for promoting crystallization of an amorphous silicon film or a compound containing the catalytic element is held in contact with the amorphous silicon film. Under this condition, the amorphous silicon film is heated at a relatively low temperature of 450 to 650xc2x0 C., e.g., about 550xc2x0 C., to crystallize the amorphous silicon film totally or partially. The silicon film is annealed at a higher temperature to promote further crystallization. For example, where the substrate is made of quartz, the anneal is conducted at about 1000xc2x0 C. In this way, a crystalline silicon film of quite high crystallinity is obtained.
A useful method of introducing the catalytic element for promoting crystallization is a method of applying a solution containing the catalytic element to the surface of the amorphous silicon film.
The present invention is characterized in that the catalytic element is introduced while kept in contact with the surface of the amorphous silicon film. This is quite important where the amount of the catalytic element is controlled.
The catalytic element may be introduced either into the top surface or into the bottom surface of the amorphous silicon film. Where the catalytic element is introduced into the top surface of the amorphous silicon film, a solution containing the catalytic element is applied to the surface of the amorphous silicon film after the amorphous silicon film is formed. Where the catalytic element is introduced into the bottom surface of the amorphous silicon film, the solution containing the catalytic element is applied to the surface of a base layer and the catalytic element is held in contact with the surface of the base layer before the amorphous silicon film is formed.
In another feature of the invention, an active region of a semiconductor device having at least one of PN, PI, NI, and other junctions is formed, using a crystallized silicon film. Examples of the semiconductor device include TFTS, diodes, and photosensors.
The following advantages can be obtained by adopting the novel structure:
(a) The concentration of the catalytic element in the solution can be controlled accurately in advance. The crystallinity can be enhanced. Also, the amount of the catalytic element can be reduced.
(b) If the solution is in contact with the surface of the amorphous silicon film, the amount of the catalytic element introduced into the amorphous silicon film is determined by the concentration of the catalytic element in the solution.
(c) Since the catalytic element adsorbed onto the surface of the amorphous silicon film principally contributes to the crystallization, the catalytic element can be introduced with a minimum concentration.
(d) Since a high temperature process is not needed, a crystalline silicon film of good crystallinity can be obtained.
Where the method of applying to the surface of the amorphous silicon film the solution containing the element for promoting the crystallization is used, a water solution, an organic solvent solution, or the like can be used as the above described solution. xe2x80x9cContainingxe2x80x9d means that the catalytic element is contained as a compound, otherwise the catalytic element is contained by simply dispersion.
The solution containing the catalytic element can be selected from water, alcohols, acids, and ammonia which are polar solvents.
Where nickel is used as a catalyst and contained in a polar solvent, the nickel is introduced as a nickel compound. Typically, this nickel compound is selected from nickel bromide, nickel acetate, nickel oxalate, nickel carbonate, nickel chloride, nickel iodide, nickel nitrate, nickel sulfate, nickel formate, nickel acetylacetonate, 4-cyclohexyl nickel butanoate, nickel oxide, and nickel hydroxide.
The solvent containing the catalytic element is selected from benzene, toluene, xylene, carbon tetrachloride, chloroform, and ether which are nonpolar solvents.
In this case, the nickel is introduced as a nickel compound. Typically, this nickel compound is selected from nickel acetylacetonate and 2-ethylhexyl nickel.
It is useful to add a surface active agent to the solution containing the catalytic element. This is intended to enhance the adhesion to the added surface and to control the adsorption. This surface active agent may be previously applied to the surface.
Where single nickel is used as the catalytic element, it is necessary to dissolve it in an acid.
In the example described above, a solution in which nickel as a catalytic element is fully dissolved is used. It is not always necessary that nickel should be fully dissolved. In this case, a material such as emulsion which is obtained by uniformly dispersing powder either of single nickel or of a nickel compound in a dispersing medium may be used. Also, a solution adapted for formation of an oxide film may be employed. OCD (Ohka diffusion source) manufactured by Tokyo Ohka Industrial Ltd., can be used as this solution. If this OCD solution is used, a silicon oxide film can be easily formed by applying the OCD solution to the surface to be applied and baking the surface at about 200xc2x0 C. Furthermore, adding of impurities can be used in the present invention.
These principles can be applied also where materials other than nickel are used as the catalytic element.
Where nickel is used as the catalytic element for promoting crystallization and a polar solvent such as water is used as the solution containing this nickel, if this solution is applied directly to an amorphous silicon film, the solution might be repelled. In this case, a thin oxide film having a thickness of less than 100 xc3x85 is first formed. A solution containing a catalytic element is applied to its surface. In this way, the solution can be applied uniformly. Where a material such as a surface active agent is added to the solution, the wettability can be effectively improved.
Where a nonpolar solvent such as 2-ethylhexyl nickel is used as the solution, it can be directly applied to the surface of the amorphous silicon film. In this case, it is advantageous to previously apply a material such as an intimate contact agent used for application of a resist. However, if the amount of the applied material is too large, then addition of the catalytic element to the amorphous silicon film will be hindered.
Although the amount of the catalytic element in the solution depends on the kind of the solution, the ratio of the weight of nickel to the weight of the solution is preferably about 200 to 1 ppm, more preferably 50 to 1 ppm (by weight calculation). This value is determined, in accordance with the concentration of the nickel in the crystallized film and the resistance to hydrofluoric acid.
The heating temperature used during the crystallization process is set to 450 to 650xc2x0 C. in the present invention. This is important for the following reason. As described previously, in the present invention, crystallization is started only at the interface between the catalytic element and the amorphous thin silicon film to obtain a thin film of crystalline silicon which has high crystallinity and homogeneous particle diameters. If nuclei form or crystals grow from locations other than the interface, the characteristics are made nonuniform, producing undesirable results. Our experiments have demonstrated that if the temperature lies in the above described range of 450-650xc2x0 C. and if this process is conducted in a short time, crystallization in portions which is not in contact with the catalytic element can be neglected, and that configuration according to the present invention can be obtained. Where the temperature is below the above described range, the crystals do not grow sufficiently even if the catalytic element is added. Conversely, where the temperature is above the range described above, the crystals grow irrespective of the presence of a catalytic element.
After the crystallizing processing, an anneal can be carried out at a higher temperature. This improves the characteristics at the interface (boundary) of crystal grains and enhances the crystallinity of the crystallized silicon film. Moreover, the amorphous portion can be completely eliminated from the silicon film by carefully controlling the conditions in this process. In this manner, the degradation of the amorphous silicon film can be effectively prevented from proceeding with the passage of time. If this process is not carried out, high barriers are created at the grain boundaries and other problems occur. Typically, high mobilities cannot be obtained. With respect to reliability, it is difficult to form a stable device because of the effects of a trace amount of the catalytic elements bonded to amorphous portion existing at the grain boundaries.
Considering the atmospheric control during the high temperature annealing, a conventional process for thermally annealing a semiconductor is performed under an inert gas such as nitrogen. However, in the case of annealing a crystalline silicon film obtained by addition of a catalyst element and low temperature crystallization, it has been found extremely effective to conduct the high temperature annealing under an oxidizing gas atmosphere such as of oxygen, thereby to obtain the crystalline silicon film having stable characteristics. The reason for the above finding is yet to be clarified; presumably, the bonds between the catalyst element and silicon which are present in large quantity in the amorphous silicon portion become newly form bonds with oxygen to form a stable structure.
A silicon oxide film produced by thermal oxidation can also be formed by the aforementioned high temperature annealing of the crystalline silicon film in an oxidizing atmosphere, thereby improving its crystallinity. The silicon oxide film is very dense. The silicon oxide film having a thickness of several hundreds of angstroms or more is found to be sufficiently reliable to use as a gate insulating film. However, a high strain generates at the boundary between the crystalline silicon and the thermally oxidized film. it is therefore preferred to form the thermally oxidized film such thin as possible. Accordingly, to eliminate the degradation of the characteristics due to the presence of the strain, the thermally oxidized film once formed during the high temperature annealing in an oxidizing atmosphere performed for increasing the crystallinity of the silicon film may be etched to form a newly gate insulating film.
Further, the structure can be modified. That is, the amount of the catalytic element is reduced drastically, and crystallization caused by the first heating is ended immediately after generation of nuclei. Then, the crystals are grown by a high temperature anneal. In this case, the process for generating nuclei is effected independent of the process for growing the crystals. These processes are carried out at their respective appropriate temperatures.
The high temperature anneal for improving the crystallinity can be an ordinary anneal conducted within an electric furnace. Also, a method for irradiating intense light, especially infrared light, can be adopted. Infrared light is not readily absorbed by glass but easily absorbed by a thin silicon film. Therefore, a thin silicon film formed on a glass substrate can be selectively heated. This method using infrared radiation is called rapid thermal anneal (RTA) or rapid thermal process (RTP).
Also, crystals can be selectively grown by selectively applying a solution containing a catalytic element. In this case, it is possible that the crystals grow from the applied regions toward the non-applied regions substantially parallel to the surface of the silicon film. The regions in which the crystals grow substantially parallel to the surface of the silicon film are referred to herein as the lateral crystal growth regions.
It is confirmed that the concentration of the catalytic element is low in these regions where the crystals are grown laterally. Although it is advantageous to use a crystalline silicon film as the active region of a semiconductor device, the concentration of impurities in the active region should generally be lower. Accordingly, formation of the active region of a semiconductor device using the lateral crystal growth regions is useful for fabrication of a device.
In the present invention, the best effects can be produced when nickel is used as a catalytic element. Other desirable catalytic elements are Ni, Pt, Cu, Ag, Au, In, Sn, Pd, P, As, and Sb. Also, the catalytic element can be one or more elements selected from the groups VIII, IIIb, IVb, and Vb elements of the periodic table.
The method of introducing a catalytic element is not limited to use of a solution such as water solution or an alcohol. Various substances containing a catalytic element can be used. For example, a metal compound or oxide containing a catalytic element can be used.